1. Field of the Invention
The invention relates to a transparent, biaxially oriented polyester film with a base layer B which is composed of at least 80% by weight of a thermoplastic polyester, and with at least one outer layer A. Another polymeric layer made from a copolymer crosslinked using an acrylic and/or methacrylic compound, has been applied to that side of the film opposite to the outer layer A. The film has low oxygen transmission and exhibits very good adhesion between the respective layers. The coated film moreover has low coefficient of friction, excellent windability, and excellent antistatic performance. It is particularly suitable for packaging purposes, specifically for packaging foodstuffs or other consumable items. The invention further relates to a process for the production of the film.
2. Description of the Related Art
EP-A-0 878 297 describes a transparent, biaxially oriented polyester film with a base layer B which is composed of at least 80% by weight of a thermoplastic polyester, and with an outer layer A. The latter is composed of a mixture of polymers which contains at least 40% by weight of ethylene 2,6-naphthalate units and up to 40% by weight of ethylene terephthalate units, and/or up to 60% of units made from cycloaliphatic or aromatic diols and/or dicarboxylic acids. The film has low oxygen transmission, not more than 80 cm3/(m2 bar d). If the outer layer A of the film contains large amounts of ethylene 2,6-naphthalate units, the film tends to delaminate between the outer layer A and the base layer B. In contrast, if the outer layer A contains low concentrations of ethylene 2,6-naphthalate units, the layer thickness has to be raised in order to achieve the desired oxygen transmission.
According to Example 8 of EP-A-0 878 297, the outer layer A of a film uses pure polyethylene 2,6-naphthalate (PEN) (corresponds to 100% by weight of ethylene 2,6-naphthalate units). In this case there is only slight adhesion between the outer layer A and the base layer B. The film is unsuitable for practical use (e.g. as a composite film), because the bond releases even under a small mechanical load.
According to Example 11 of EP-A-0 878 297, the outer layer A of a film contains 60% by weight of ethylene 2,6-naphthalate units. To achieve the desired low oxygen transmission, less than 80 cm3/(m2 bar d), the thickness of the outer layer A has to be raised to 3 μm, this being economically disadvantageous (high capital expenditure and high material costs).
The film has other disadvantages relating mainly to windability and processability during metallizing. For example, prior art films tend to become electrostatically charged during metallizing/ceramic coating, and tend to block after metallizing/ceramic coating. The printability of the film is also not ideal, in particular when polyvinyl butyral (PVB) inks are utilized.
U.S. Pat. No. 5,795,528 describes a coextruded film laminate which has alternating layers made from PEN and polyethylene terephthalate (PET). Like the film of EP-A-0 878 297, the film tends to delaminate between the individual layers made from PEN and PET, since there is only low adhesion. This type of laminate is therefore likewise unsuitable for practical use.